Voltage controlled oscillation device

ABSTRACT

A voltage controlled oscillation device includes a voltage controlled oscillator, fixed-frequency oscillator, frequency mixer, and frequency selector. The voltage controlled oscillator changes the output signal frequency in the microwave band in accordance with the input voltage of a frequency control signal. The fixed-frequency oscillator has a fixed oscillation frequency higher than that of the voltage controlled oscillator. The frequency mixer mixes the output signal from the fixed-frequency oscillator and the output signal from the voltage controlled oscillator and outputs the sum frequency and difference frequency between the two signals. The frequency selector selects and outputs one of the sum frequency and difference frequency contained in the output signal from the frequency mixer.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a voltage controlled oscillationdevice and, more particularly, to a voltage controlled oscillationdevice which can be suitably applied to a PLL (Phase Locked Loop)circuit of a microwave communication device.

[0002] In recent years, large-capacity data communications usingmicrowave communication channels have been executed between even compactterminal apparatuses. Accordingly, use of multilevel transmissionsignals have been promoted in order to effectively use the radio waveresources.

[0003] Multilevel radio transmission signals readily cause transmissionerrors due to the influence of phase noise. For transmission apparatusesfor transmitting multilevel radio signals, the phase noise of a localoscillator in a frequency converter is required to be reduced to improvethe transmission quality. In addition, to effectively use free frequencychannels, a local oscillator is required to allow its oscillationfrequency to change over a wider frequency band.

[0004] A fixed-frequency oscillator or a voltage controlled oscillatorcapable of controlling the frequency in accordance with an externalcontrol voltage is generally used as a local oscillator. An example of aconventional fixed-frequency low-phase-noise microwave oscillator is adielectric oscillator using a dielectric resonator. The dielectricoscillator is constituted by an oscillation active element and adielectric resonator. As a characteristic feature of the dielectricresonator, since the Q factor (Quality factor) is high, phase noise nearthe carrier of the oscillator output is low.

[0005] In the dielectric oscillator, however, the oscillation frequencyis determined by the resonance frequency of the dielectric resonator.The resonance frequency of the dielectric resonator readily varies dueto the electromagnetic coupling to peripheral elements or a change intemperature. For this reason, the dielectric oscillator has a problemthat the oscillation frequency is less stable than, e.g., a quartzoscillator.

[0006] An arrangement has been proposed in which the output signal froma voltage controlled oscillator is synchronized with an oscillator suchas a quartz oscillator having a stable frequency using a PLL circuit,thereby improving the stability of oscillation frequency. The voltagecontrolled oscillator in this arrangement is generally constituted by anoscillation active element, a resonator, and a varactor diode coupled tothe resonator. In this arrangement, the reverse bias voltage of thevaractor diode is controlled to change the reactance, therebycontrolling the output frequency of the oscillator.

[0007] In the voltage controlled oscillator with the above arrangement,since the phase noise can be made low as the Q factor of the resonatorbecomes high, a dielectric resonator is often used in a microwaveoscillator. An example is a voltage controlled oscillator disclosed inJapanese Patent Laid-Open No. 9-191215 (reference 1). The voltagecontrolled oscillator described in reference 1 comprises a tuningcircuit section capable of changing the tuning frequency, an oscillatorsection, and a high-pass filter inserted therebetween.

[0008] The oscillator section is designed to have a capacitive inputimpedance in its oscillation frequency band. The high-pass filter on theinput side is designed to conversely cause a phase lead, therebycompensating for the capacitive input impedance of the oscillatorsection. With this arrangement, a voltage controlled oscillator whichsuppresses the shift between the oscillation frequency and the resonancefrequency of the dielectric resonator and can change the oscillationfrequency is obtained.

[0009] It is also important for a voltage controlled oscillator toensure a wide frequency variable band. As a means for widening theoscillation frequency variable band, a structure which strengthenscoupling between the resonator and the varactor diode is generallyemployed. However, since the Q factor of the varactor diode is not high,the total Q factor becomes low as the coupling to the resonator isstrengthened, resulting in degradation in phase noise characteristicnear the carrier. When the Q factor of the resonator is made high inorder to improve the phase noise characteristic, the frequency variableband becomes narrow, resulting in difficulty in widening the frequencyband.

[0010] Since the voltage controlled oscillator disclosed in reference 1employs the arrangement for controlling the oscillation frequency usingthe varactor diode, the low phase noise and wide frequency variable bandcannot be simultaneously realized, as described above. The low phasenoise and wide frequency variable band in the voltage controlledoscillator have tradeoff relationships and can hardly be simultaneouslyrealized.

[0011] To simultaneously realize the low phase noise and wide frequencyvariable band in the voltage controlled oscillator in the microwaveband, the following arrangements can be used, though they have problems.

[0012] As the first arrangement, a low-phase-noise element such as aquartz oscillator is used as a reference signal source for phasecomparison. With this arrangement, the phase noise of the voltagecontrolled oscillator can be suppressed to the phase noise level of thereference signal source at maximum in a band (loop band) correspondingto a feedback gain of 1 or more in the loop. However, since the phasenoise can be suppressed by the loop only in the loop band, the phasenoise characteristics cannot be improved in a detuning frequency bandfalling outside the loop band.

[0013] As the second arrangement, a PLL circuit using an integerfrequency divider can be used. In this arrangement, however, the phasecomparison frequency must be equal to or lower than the minimum settablefrequency interval (step frequency) of the output frequency. Inaddition, the loop band cannot be set beyond the phase comparisonfrequency. Hence, when the step frequency is low, the phase noise canhardly be suppressed in a wide band by the PLL circuit.

[0014] As the third arrangement, a fractional frequency divider ordirect digital synthesizer circuit can be combined with a PLL circuit.With this arrangement, a low step frequency and wide loop band can beconsistent. However, the circuit of this type generally has a digitalcalculator which rounds the values at digit positions lower than thesignificant digit position of a calculation result and therefore alwaysgenerates a small error. Such small errors are accumulated every timecalculation is repeated, and canceled when the magnitude has reached thesignificant digit position, resulting in a spurious low frequency.

[0015] As the fourth arrangement, a method has been proposed in whichfrequency variations outside the loop band of a PLL are suppressed usinga delay detector to reduce the phase noise over a wide band. An exampleis a frequency synthesizer disclosed in Japanese Patent Laid-Open No.9-321619 (reference 2). The frequency synthesizer disclosed in reference2 comprises a delay detector for detecting the delay of the outputsignal, a phase locked loop (PLL), and a coupling circuit for couplingthe output signals to output the control voltage for the voltagecontrolled oscillator. The coupling circuit comprises a first low-passfilter for a PLL circuit output signal, a high-pass filter for a delaydetector output signal, and a second low-pass filter for synthesizingthe output signals to generate the control voltage for the voltagecontrolled oscillator.

[0016] With this arrangement, in a high-frequency band where the phasenoise suppressing effect by the PLL becomes small, the phase noisesuppressing effect is obtained using the delay detector output signal,thereby obtaining the phase noise suppressing effect in a wider band.However, the frequency synthesizer disclosed in reference 2 has acomplex circuit arrangement, and therefore, the device undesirablybecomes bulky and expensive.

[0017] As the fifth arrangement, a multiple PLL circuit in which aplurality of PLLs are combined can be used. However, since thisarrangement increases the circuit scale in proportion to the number ofloops, the device undesirably becomes bulky and expensive, like thefourth arrangement. In addition, in this arrangement, since a pluralityof frequency components are simultaneously processed, a spuriousfrequency is generated.

[0018] As described above, in the conventional voltage controlledoscillator, the low phase noise and wide output frequency variable bandhave tradeoff relationships and can hardly be simultaneously realized.This is because when coupling between the resonator and the varactordiode is strengthened to widen the output frequency variable band, thetotal Q factor of the resonator lowers to degrade the phase noisecharacteristics. Various kinds of arrangements conventionally used havethe following problems.

[0019] As the first problem, when a low-phase-noise element such as aquartz oscillator is used as a reference signal source for phasecomparison, the phase noise characteristics cannot be improved in thedetuning frequency band falling outside the loop band. This is becausethe phase noise can be suppressed by the loop only in the loop band.

[0020] As the second problem, when a PLL circuit using an integerfrequency divider is used to prevent degradation in phase noisecharacteristics, a wide loop band can hardly be ensured when the stepfrequency is made low, so a sufficient phase noise suppressing bandcannot be obtained. This is because of the principle of the PLL circuitusing the integer frequency divider in which the loop band must be lowerthan the step frequency.

[0021] As the third problem, when a fractional frequency divider ordirect digital synthesizer is used in a PLL circuit to widen the phasenoise suppressing band, a spurious low frequency is generated. Thereason for this is that the fractional frequency divider or directdigital synthesizer uses a digital calculator, and accumulation andcancel of small errors due to rounding of the calculation result of thedigital calculator are periodically repeated.

[0022] As the fourth problem, when a delay detector is used to widen thephase noise suppressing band, the circuit scale becomes large andcomplex.

[0023] As the fifth problem, when a multiple PLL circuit which combinesa plurality of PLLs is used, the circuit scale becomes large and complexin proportion to the number of loops. In addition, since a plurality offrequency components are simultaneously processed, a spurious frequencyis generated.

SUMMARY OF THE INVENTION

[0024] It is an object of the present invention to provide a voltagecontrolled oscillation device having low phase noise and a wide outputfrequency variable band.

[0025] It is another object of the present invention to provide acompact and inexpensive voltage controlled oscillation device.

[0026] It is still another object of the present invention to provide avoltage controlled oscillation device with a reduced spurious frequency.

[0027] In order to achieve the above objects, according to the presentinvention, there is provided a voltage controlled oscillation devicecomprising voltage controlled oscillation means for changing an outputsignal frequency in a microwave band in accordance with an input voltageof a frequency control signal, fixed-frequency oscillation means havinga fixed oscillation frequency higher than that of the voltage controlledoscillation means, frequency mixing means for mixing an output signalfrom the fixed-frequency oscillation means and an output signal from thevoltage controlled oscillation means and outputting a sum frequency anddifference frequency between the two signals, and frequency selectionmeans for selecting and outputting one of the sum frequency anddifference frequency contained in the output signal from the frequencymixing means.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0028]FIG. 1 is a block diagram showing a voltage controlled oscillatoraccording to the first embodiment of the present invention;

[0029]FIG. 2 is a block diagram showing a voltage controlled oscillatoraccording to the second embodiment of the present invention; and

[0030]FIG. 3 is a block diagram showing a voltage controlled oscillatoraccording to the third embodiment of the present invention.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

[0031] The present invention will be described below in detail withreference to the accompanying drawings.

[0032]FIG. 1 shows a voltage controlled oscillator according to thefirst embodiment of the present invention. Referring to FIG. 1, thevoltage controlled oscillator of this embodiment comprises alow-phase-noise fixed-frequency oscillator 11 whose output frequency isin the microwave band, a voltage controlled oscillator 12 connected to acontrol voltage input terminal 1, a frequency mixer 13 formed from amixer for receiving the outputs from the fixed-frequency oscillator 11and voltage controlled oscillator 12, and a frequency selector 14 forreceiving the output from the frequency mixer 13.

[0033] The fixed-frequency oscillator 11 generates a fixed-frequencysignal 101. Since the fixed-frequency oscillator 11 has no component forexternally controlling the frequency, the fixed-frequency signal 101 haslower phase noise as compared to an output signal from a voltagecontrolled oscillator for generating the same frequency. Morespecifically, the fixed-frequency oscillator 11 can be constructed usinga dielectric oscillator using a dielectric resonator.

[0034] The voltage controlled oscillator 12 generates a variablefrequency signal 103 in accordance with a frequency control signal 102input to the control voltage input terminal 1. The frequency of thevariable frequency signal 103 is set to be lower than that of thefixed-frequency signal 101. For this reason, even when the outputfrequency is changed over a wide band, the phase noise can besuppressed. More specifically, the voltage controlled oscillator 12 canbe constructed using a microstripline voltage controlled oscillatorusing a microstripline resonator.

[0035] The frequency mixer 13 mixes the fixed-frequency signal 101 fromthe fixed-frequency oscillator 11 and the variable frequency signal 103from the voltage controlled oscillator 12 and outputs a signal. Anoutput signal 104 from the frequency mixer 13 contains the sum frequencycomponent and difference frequency component between the fixed-frequencysignal 101 and variable frequency signal 103.

[0036] The frequency selector 14 selects a desired frequency componentof the sum frequency component and difference frequency componentcontained in the output signal 104 from the frequency mixer 13 andoutputs an output signal 105 to a signal output terminal 2. The reasonwhy the output frequency band can be widened while suppressing the phasenoise low will be described below in more detail.

[0037] The dielectric oscillator exemplified as the fixed-frequencyoscillator 11 shown in FIG. 1 and the microstripline voltage controlledoscillator exemplified as the voltage controlled oscillator 12 are knownwell by those skilled in the art. A case wherein a center frequency of12.5 [GHz] of the output signal 105 is obtained using, as examples ofoscillation frequencies of these devices, a dielectric oscillator havingan oscillation frequency of 11 [GHz] as the fixed-frequency oscillator11 and a microstripline voltage controlled oscillator having anoscillation frequency of 1.5 [GHz] as the voltage controlled oscillator12 will be described.

[0038] The frequency mixer 13 uses a microwave band mixer, and thefrequency selector 14 uses a filter, both of which are known well bythose skilled in the art, and a detailed description thereof will beomitted.

[0039] The output signal from the dielectric oscillator having anoscillation frequency of 11 [GHz], which has a sufficiently low phasenoise level, is widely supplied for the application purpose of amicrowave band local oscillator for performing data communication usingmultilevel radio signals. On the other hand, since the microstriplinevoltage controlled oscillator having an oscillation frequency of 1.5[GHz] can change its oscillation frequency, the phase noise level ishigher than that of, e.g., a dielectric oscillator having an oscillationfrequency of 1.5 [GHz].

[0040] However, many such microstripline voltage controlled oscillatorshave been supplied, which have an oscillation frequency variable bandfrom about 100 [MHz] to 200 [MHz] within the phase noise level rangeallowable for the application purpose of the microwave band localoscillator for performing data communication using multilevel radiosignals.

[0041] The frequency mixer 13 shown in FIG. 1 mixes the output signal101 from the fixed-frequency oscillator 11 and the output signal 103from the voltage controlled oscillator 12 and outputs the sum frequencycomponent and difference frequency component. For this reason, theoutput signal 104 contains signals with frequencies of 12.5 [GHz] and9.5 [GHz]. When the sum frequency component is selected by the frequencyselector 14, the signal having a frequency of 12.5 [GHz] is obtainedfrom the output terminal 2.

[0042] When the output signal from the voltage controlled oscillator 12is changed 200 [MHz] with respect to 1.5 [GHz] by controlling thefrequency control signal 102 input to the control voltage input terminal1, this change in frequency is directly reflected to the frequency ofthe output signal 105. More specifically, the frequency of the outputsignal 105 from the output terminal 2 can be changed in a wide band of200 [MHz] with respect to 12.5 [GHz] by changing the frequency controlsignal 102 input to the control voltage input terminal 1.

[0043] As described above, the oscillator shown in FIG. 1 constitutes avoltage controlled oscillator whose output signal 105 is changed in thewide band of 200 [MHz] with respect to 12.5 [GHz] as a whole. Adielectric voltage controlled oscillator having an oscillation frequencyof 12.5 [GHz] has also generally been provided as a voltage controlledoscillator.

[0044] However, the output frequency variable bandwidth of thedielectric voltage controlled oscillator having an oscillation frequencyof 12.5 [GHz] is as narrow as 5 [MHz] to 10 [MHz], so the oscillationfrequency variable band is not sufficient. This is because thedielectric resonator in the dielectric voltage controlled oscillator hasa high Q factor, and it is therefore difficult to change the resonancefrequency by an external varactor diode or the like in the same manneras in a microstripline resonator.

[0045] The phase noise level of the dielectric voltage controlledoscillator having an oscillation frequency of 12.5 [MHz] is higher thanthat of a fixed-frequency oscillator using the same dielectricresonator. This is because since an element having a low Q factor iscoupled to the resonator, the total Q factor lowers, and the phase noiselevel rises.

[0046] As another example of the voltage controlled oscillator, amicrostripline voltage controlled oscillator having an oscillationfrequency of 12.5 [GHz] has also generally been provided. However, thephase noise level of the microstripline voltage controlled oscillatorhaving an oscillation frequency of 12.5 [GHz] is higher than that of thedielectric voltage controlled oscillator by about 10 [dB]. This isbecause the Q factor of the microstripline resonator in themicrostripline voltage controlled oscillator is lower than that of thedielectric resonator. More specifically, when the oscillation frequencybecomes high, the influence of noise generated by the oscillation activeelement generally becomes large, and as the Q factor of the resonatorincreases, the phase noise level due to its influence becomes high.

[0047] According to the voltage controlled oscillator of the presentinvention, an arrangement is employed, in which the output signal fromthe fixed-frequency oscillator having a high oscillation frequency andlow phase noise level and the output signal from the voltage controlledoscillator having a low output frequency and wide output frequencyvariable band are mixed, and a necessary frequency wave is selected andoutput. Thus, a low-phase-noise output signal and a wide outputfrequency variable band are simultaneously attained.

[0048]FIG. 2 shows a voltage controlled oscillator according to thesecond embodiment of the present invention. In this embodiment,small-scale circuits are added to the basic arrangement of the firstembodiment, thereby obtaining a puerility of output signals.

[0049] The voltage controlled oscillator shown in FIG. 2 comprises alow-phase-noise fixed-frequency oscillator 11 whose output frequency isin the microwave band, a first voltage controlled oscillator 12 aconnected to a control voltage input terminal 1 a, a second voltagecontrolled oscillator 12 b connected to a control voltage input terminal1 b, a first frequency mixer 13 a for receiving the outputs from thefixed-frequency oscillator 11 and voltage controlled oscillator 12 a, asecond frequency mixer 13 b for receiving the outputs from thefixed-frequency oscillator 11 and second voltage controlled oscillator12 b, a first frequency selector 14 a for receiving the output from thefrequency mixer 13 a, and a second frequency selector 14 b for receivingthe output from the frequency mixer 13 b.

[0050] The voltage controlled oscillators 12 a and 12 b output outputsignals 103 a and 103 b in accordance with control voltages 102 a and102 b input from the control voltage input terminals 1 a and 1 b,respectively. The output signals 103 a and 103 b are input to thefrequency mixers 13 a and 13 b and mixed with an output signal 101 fromthe fixed-frequency oscillator 11.

[0051] The frequency mixers 13 a and 13 b generate output signals 104 aand 104 b, each containing a sum frequency component and differencefrequency component, and output them to the frequency selectors 14 a and14 b, respectively. The frequency selectors 14 a and 14 b select desiredfrequency components contained in the output signals 104 a and 104 bfrom the frequency mixers 13 a and 13 b and output output signals 105 aand 105 b to signal output terminals 2 a and 2 b, respectively.

[0052] According to this embodiment, a voltage controlled oscillatorwhich can obtain a plurality of output signals with a small-scalecircuit arrangement and reduce cost by decreasing the number ofexpensive dielectric oscillators can be provided. In this embodiment,two output signals 105 a and 105 b are output. However, an arrangementfor outputting three or more output signals can also be employed.

[0053]FIG. 3 shows a voltage controlled oscillator according to thethird embodiment of the present invention. In this embodiment, thearrangement of the first embodiment is further improved to widen theoutput signal frequency variable band.

[0054] The voltage controlled oscillator shown in FIG. 3 comprises alow-phase-noise fixed-frequency oscillator 11 whose output frequency isin the microwave band, first and second voltage controlled oscillators12 a and 12 b connected to a control voltage input terminal 1, aswitching device 15 which is connected to a switching signal inputterminal 3 and receives the outputs from the voltage controlledoscillators 12 a and 12 b, a frequency mixer 13 for receiving theoutputs from the fixed-frequency oscillator 11 and switching device 15,and a frequency selector 14 for receiving the output from the frequencymixer 13.

[0055] The voltage controlled oscillators 12 a and 12 b output outputsignals 103 a and 103 b, respectively, in accordance with a controlvoltage 102 input to the control voltage input terminal 1. The switchingdevice 15 receives the two output signals 103 a and 103 b from thevoltage controlled oscillators 12 a and 12 b, selects one of the inputsignals in accordance with a switching control signal 106 input from theswitching signal input terminal 3, and outputs the signal as an outputsignal 107.

[0056] The frequency mixer 13 mixes an output signal 101 from thefixed-frequency oscillator 11 with the output signal 107 from theswitching device 15 and outputs an output signal 104. The output signal104 from the frequency mixer 13 contains the sum frequency component anddifference frequency component between the two input signals. Thefrequency selector 14 selects a desired frequency component from theoutput signal 104 from the frequency mixer 13 and outputs an outputsignal 105 to a signal output terminal 2.

[0057] According to this embodiment, adjacent output frequency bands canbe individually set for the voltage controlled oscillators 12 a and 12b. For this reason, when the switching device 15 is switched as needed,a wider output band than the individual frequency variable bands of thevoltage controlled oscillators 12 a and 12 b can easily be obtained.

[0058] In this embodiment, output signals from two voltage controlledoscillators are switched. However, an arrangement for switching outputsignals from three or more voltage controlled oscillators can also beemployed as needed.

[0059] As has been described above, according to the present invention,the output signal from the fixed-frequency oscillator having a highoscillation frequency and low phase noise level and the output signalfrom the voltage controlled oscillator having a low oscillationfrequency and wide output frequency variable band are mixed, and anecessary frequency wave is selected and output. Hence, a voltagecontrolled oscillator which has a wide output frequency variable bandwhile suppressing the phase noise can be provided.

[0060] In addition, since a simple arrangement using elements well-knownto those skilled in the art is employed, the device can easily be madecompact and inexpensive. When a small-scale arrangement is added, avoltage controlled oscillator capable of simultaneously obtaining outputsignals of a plurality of kinds can be provided.

[0061] Furthermore, when a small-scale arrangement is added, a voltagecontrolled oscillator having a wider output frequency variable band canbe provided. The voltage controlled oscillator according to the presentinvention has sufficiently low phase noise and therefore requires noarrangement for suppressing the phase noise. Hence, the problem of thespurious frequency generated by the phase noise suppressing circuit canbe eliminated.

What is claimed is:
 1. A voltage controlled oscillation devicecomprising: voltage controlled oscillation means for changing an outputsignal frequency in a microwave band in accordance with an input voltageof a frequency control signal; fixed-frequency oscillation means havinga fixed oscillation frequency higher than that of said voltagecontrolled oscillation means; frequency mixing means for mixing anoutput signal from said fixed-frequency oscillation means and an outputsignal from said voltage controlled oscillation means and outputting asum frequency and difference frequency between the two signals; andfrequency selection means for selecting and outputting one of the sumfrequency and difference frequency contained in the output signal fromsaid frequency mixing means.
 2. A device according to claim 1, whereinsaid fixed-frequency oscillation means comprises a dielectric oscillatorusing a dielectric resonator.
 3. A device according to claim 1, whereinsaid voltage controlled oscillation means comprises a microstriplinevoltage controlled oscillator using a microstripline resonator.
 4. Avoltage controlled oscillation device comprising: first voltagecontrolled oscillation means for changing an output signal frequency ina microwave band in accordance with an input voltage of a firstfrequency control signal; second voltage controlled oscillation meansfor changing an output signal frequency in the microwave band inaccordance with an input voltage of a second frequency control signal;fixed-frequency oscillation means having a fixed oscillation frequencyhigher than that of said first and second voltage controlled oscillationmeans; first frequency mixing means for mixing an output signal fromsaid fixed-frequency oscillation means and an output signal from saidfirst voltage controlled oscillation means and outputting a sumfrequency and difference frequency between the two signals; secondfrequency mixing means for mixing the output signal from saidfixed-frequency oscillation means and an output signal from said secondvoltage controlled oscillation means and outputting a sum frequency anddifference frequency between the two signals; first frequency selectionmeans for selecting and outputting one of the sum frequency anddifference frequency contained in the output signal from said firstfrequency mixing means; and second frequency selection means forselecting and outputting one of the sum frequency and differencefrequency contained in the output signal from said second frequencymixing means.
 5. A device according to claim 4, wherein saidfixed-frequency oscillation means comprises a dielectric oscillatorusing a dielectric resonator.
 6. A device according to claim 4, whereineach of said first and second voltage controlled oscillation meanscomprise microstripline voltage controlled oscillators each using amicrostripline resonator.
 7. A voltage controlled oscillation devicecomprising: first and second voltage controlled oscillation means forchanging output signal frequencies in a microwave band in accordancewith an input voltage of a common frequency control signal; switchingmeans for selecting and outputting one of output signals from said firstand second voltage controlled oscillation means in accordance with aswitching control signal; fixed-frequency oscillation means having afixed oscillation frequency higher than that of said first and secondvoltage controlled oscillation means; frequency mixing means for mixingan output signal from said fixed-frequency oscillation means and anoutput signal from said switching means and outputting a sum frequencyand difference frequency between the two signals; and frequencyselection means for selecting and outputting one of the sum frequencyand difference frequency contained in the output signal from saidfrequency mixing means.
 8. A device according to claim 7, wherein saidfixed-frequency oscillation means comprises a dielectric oscillatorusing a dielectric resonator.
 9. A device according to claim 7, whereineach of said first and second voltage controlled oscillation meanscomprise microstripline voltage controlled oscillators each using amicrostripline resonator.
 10. A device according to claim 7, wherein theoutput frequency bands of said first and second voltage controlledoscillation means are set to be adjacent to each other.